How does the Professor demonstrate logic? How does this demonstration enable him - and us - to believe in Narnia?

The Professor teaches Peter and Susan the importance of logic, but surprises them by not rejecting the seemingly "logical" assumption that Lucy's story about Narnia is untrue. Instead, he considers the reasonable possibility that alternate worlds might, in fact, exist. In the end, he comes to thoroughly believe the children's story about their journey through Narnia and all that happened there, but advises them to keep the story close to their hearts and not to talk about it too openly.